1. Field of the Invention
The present. invention relates to a drum brake device for vehicles. More particularly, the present invention relates to improvements of a control lever, interposed between a wheel cylinder and a brake shoe, for distributing an anchor counter force of the brake shoe to a control piston of a wheel cylinder and an anchor pin.
Furthermore, the present invention relates to a drum brake device which is able to stabilize a braking effect in both the forward and reverse rotation directions of a brake drum.
2. Description of the Related Art
Various types of drum brake devices have been used for braking vehicles and others. Generally, the drum brake device may be categorized into brake devices of the leading trailing type, the two-leading type and the duo-servo type depending on the layout of brake shoes disposed within and along the inner circumferential surface of a cylindrical brake drum.
The duo-servo drum brake device is constructed such that a pair of brake shoes, primary and secondary brake shoes, are oppositely disposed within a cylindrical brake drum, a wheel cylinder used for expanding the brake shoes is disposed between first opposed ends of the brake shoes, and the second opposed ends of the brake shoes are link-coupled to each other.
The primary shoe is arranged such that the input of the primary shoe when viewed in the forward rotation direction of the brake drum is used as a power input, the output of the primary shoe when viewed in the same direction is coupled to an adjuster and then the input of the secondary shoe. The output of the secondary shoe is brought into contact with an anchor portion mounted on a backing plate so that the anchor portion receives the anchor counter force acting on the primary and secondary shoes.
With such a structure, when the primary and secondary shoes are expanded into braking engagement with the inner circumferential surface of the rotary brake drum, an anchor counter force acting on the primary shoe is input to the secondary shoe, and the secondary shoe is pressed against the drum inner surface. Accordingly, both of the primary and secondary shoes function as a leading shoe. The duo-servo drum brake device produces a braking force at considerably high gain.
The drum brake device of the duo-servo type is superior in many points to the brake devices of the leading trailing type and the two-leading type. For example, the former brake device is capable of producing much higher braking force than the latter brake devices. Further, size reduction of the brake device is easier and the assembling of a parking brake mechanism into the brake device per se is easier than the latter ones.
However, one draw-back of the duo-servo drum brake device is its sensitivity to variations of the friction coefficients of the linings of the brake shoes. With this, the braking force produced is likely to be unstable. The market has longed for a decisive solution to this problem.
For this technical background, the applicant of the present patent application has already proposed a duo-servo drum brake device capable of stabilizing a braking force produced thereby. In the drum brake device, a wheel cylinder includes a drive piston, a control piston, and a control lever. The drive piston presses the brake shoes in accordance with a hydraulic pressure supplied to a pressure chamber. The control piston controls the supply of hydraulic pressure to the pressure chamber. The control lever is disposed between the wheel cylinder and one of the brake shoes, and distributes an anchor counter force received from the brake shoe to an anchor pin and the control piston.
In the brake device thus constructed, the anchor counter force is transmitted from the brake shoe through the control lever to the control piston, and then the control piston is operated by the received anchor counter force. When the anchor counter force is varied to reach a predetermined magnification ratio, the hydraulic pressure supply to the pressure chamber is stopped, thereby suppressing an increase of the anchor counter force. The duo-servo drum brake device succeeds in solving the problem of the duo-servo drum brake device producing high braking effect, viz., the braking force produced is unstable.
In this drum brake device, the control lever includes a piston contact portion for contact with the control piston, an anchor contact portion for contact with the anchor portion and a shoe contact portion for contact with the brake shoe. contact portions Manufacturing the requires extensive labor. The result is to increase the cost of making the contact, to increase the cost of making the control lever, and consequently to increase the cost of manufacturing the resultant brake device.
To secure a strength of the control lever high enough to withstand the anchor counter force, the size of the control lever needs to be increased correspondingly. The securing of a space occupied by the control lever of the increased size within the brake device places a limit on the layout of the wheel cylinder and the brake shoes within the brake device. Inefficient assembling work of the brake device results.
The use of the contact portions of the control lever entails an unstable operation accuracy of the control lever caused by the anchor counter force, and hence degradation of the control accuracy of the braking effect.
Furthermore, a braking force produced by the brake device of this type when the brake drum rotates in the forward direction is equal to that produced by the same when the drum rotates in the reverse direction. Therefore, in designing the drum brake device capable of producing a stable braking force, it is important that braking force stabilizing effect equally acts irrespective of the rotation directions of the brake drum.